1. Field of the Invention
The present invention relates generally to an acceleration transducer. More particularly, the present invention relates to an electromagnetic accelerometer. Still more particularly, the present invention relates to an electromagnetic accelerometer for use in implantable medical devices.
2. Description of the Related Art
Acceleration transducers, commonly refer to as accelerometers, generate an electrical output signal corresponding to an acceleration experienced by the transducer caused by an externally applied force. Typically, the magnitude of the voltage of the electrical output signal is portional to the acceleration. Thus, by monitoring the magnitude of the voltage of the transducer's output signal, a measure of the acceleration experienced by the transducer can be determined.
Accelerometers are used in a variety of applications such as in some implantable medical devices including pacemakers and defibrillators (collectivity referred hereafter simply as pacemakers). Pacemakers use accelerometers for several reasons. First, an accelerometer may be included as a body activity sensor within the pacemaker's housing or "can" which encloses and seals the electronic circuitry of the pacemaker. An accelerometer-based body activity sensor provides an output signal that is proportional to the overall motion of the patient's body. The output signal from such a body activity sensor can be processed to distinguish a sleeping patient, for example, from a patient engaging in strenuous exercise with a high level of body motion. Rate-responsive pacemakers increase or decrease the rate of pacing (i.e., the rate at which the pacemaker emits electrical pulses to force a chamber of the heart to beat) in response to the measured index. A body activity accelerometer can be used to provide a control signal for a rate-responsive pacemaker keyed to body motion. Using a body activity accelerometer, a rate-responsive pacemaker can determine when the patient is engaging in strenuous exercise, and accordingly, increase the rate of pacing to meet the increased metabolic load of the patient during exercise. By contrast, the same rate-responsive pacemaker reduces the rate of pacing when the patient exhibits little motion such as during sleep when the patient's heart preferably beats less often. Examples of accelerometers used as body activity sensors are disclosed in U.S. Pat. Nos. 5,014,700, 5,031,615, and 5,044,366.
Accelerometers also are used for other purposes in pacemaker systems. Several attempts have been made at incorporating an accelerometer in a pacemaker lead which couples the pacemaker to the heart. A pacemaker lead typically is a thin flexible cable including one or more electrical conductors. One end of the lead couples to a header plug on the pacemaker can and the other end of the lead includes one or more conductive electrodes. A lead-based accelerometer can be used to measure the acceleration of the wall of the heart to which the lead is coupled. It is also known that lead accelerometers can be used to determine various physiological parameters such as contractility and ejection fraction. Further, an accelerometer incorporated into a lead can also be used to determine if the chamber of the heart in which an electrode is implanted has contracted in response to a pacing pulse generated by pacemaker. This determination is referred to as "capture verification."
U.S. Pat. No. 5,304,208 discloses a lead-based accelerometer coupled to an electronic pre-amplification network. This accelerometer, however, requires an extra pair of conductors to be included within the pacemaker lead to couple the accelerometer and associated electronics to the pacemaker can. It is generally recognized that there is a risk that once implanted a conductor within a lead may break disrupting the operation of the pacemaker and requiring surgical repair. Such surgery obviously increases risk and discomfort of the patient. Further, the risk of conductor breakage increases as the number of conductors in a lead increases. Reducing the number of conductors in a cardiac lead thus is highly desirable to improve the reliability of a pacemaker system. Moreover, the risk of conductor breakage in a lead is reduced if fewer conductors are included in a lead.
Many accelerometers, such as the accelerometer of U.S. Pat. No. 5,304,208, require electrical power for their operation. Implantable pacemakers and defibrillators typically operate from batteries housed within the can of the pacemaker. Because batteries store only a finite amount of electrical energy, it is highly desirable for pacemakers to use as little power as possible. Thus, pacemakers are designed for minimum power consumption. Accelerometers, such as that used in U.S. Pat. No. 5,304,208, that require electrical power for their operation are not desirable for this reason.
The output signal from an accelerometer typically requires amplification and filtering to condition the accelerometer's output signal for use by the pacemaker. Amplification and filtering circuitry requires electrical power to operate, thereby imposing an additional power drain on the pacemaker's batteries. Thus, it is desirable for transducers associated with a pacemaker, such as an accelerometer, to produce an electrical output signal that requires as little amplification and/or filtering as possible.
Accordingly, an accelerometer is needed, especially for use in implantable medical devices, that solves the shortcomings discussed above. Such an accelerometer should be small enough to be included in a typical cardiac lead coupling a pacemaker to the heart for capture verification or for determining various hemodynamic parameters. If incorporated into a lead, such an accelerometer preferably should require a minimal number of conductors in the lead. Further, such an accelerometer should require little, if any, operational power, thereby causing little or no drain on the pacemaker's battery. Battery drain could be further minimized if the accelerometer generates an output signal requiring little amplification and filtering.